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Comparison of Enhanced Organic Rankine Cycles for Geothermal Power Units
Paola Bombarda Mario Gaia Costante Invernizzi Claudio Pietra
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Exploitation of geothermal sources: approach used in this study
Focus on liquid-phase sources • exploitation of low temperature sources • avoid pollutant emissions and reduce scaling, possibly by
means of geothermal fluid pressurization • full reinjection possible
Adoption of state of the art technology + innovative cycle configuration => no relevant operating or investment risk
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
An important challenge in binary cycle design: the improvement of the heat introduction process
Several improved solutions available with respect to the base single level ORC: Multilevel evaporation cycle, Kalina cycle, supercritical cycle, zeotropic mixture cycle…….
Aim of this work: Comparison of innovative, non conventional, potentially advantageous technical solutions based on current technology
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Design criteria: best utilization of the heat exchange surface
HOT SIDE HEAT EXCHANGERS • LARGE SURFACE AREA • EXTENDED SURFACES (various kinds of finned tubes) • PLATE HEX (plate/ shell and plate/ etc) • INNOVATIVE MATRICES
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Reducing ∆Tpp :
MINOR INCREASE OF EXCHANGED THERMAL POWER MINOR INCREASE OF EVAPORATION TEMPERATURE
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Design criteria: best utilization of the heat exchange surface
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Possible improvements of the T-Q diagram, besides two level evaporation:
-”smooth” the sharp edge
-eliminate the sharp edge
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Pinch Point Smoother
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PINCH POINT SMOOTHING: a fraction of the working fluid flow evaporates at lower pressure and it is mechanically compressed to the turbine inlet pressure
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Organic Flash Cycle, arrangement: double FLASH recently proposed in literature (Ho, Mao and Greif 2012)
HEAT EXCHANGER
FLASH CYCLE: heat introduction occurs in liquid phase; vapour is generated by a flash process
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Performance evaluation: basic assumptions
Geothermal fluid temperature Variable in the range 125-150°C Reinjection temperature unconstrained Geothermal fluid volumetric flow 150 l/s Ambient temperature 15°C (as per ISO standard) Condenser cooling medium Air
Power cycle working fluid R245fa Working fluid condensation temperature
30°C
Air cooler electric parasitic consumption, percentage of condensation heat
1%
Heat Exchangers minimum ∆Tpp 2 °C isoentropic/hydraulic mechanical-electrical
Turbine efficiency 0.85 0.97 Pumps efficiency 0.75 0.95 Compressor efficiency 0.8 0.96
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Performance evaluation: selected configurations and optimization
Optimizing function: Net plant power
Reference case: • Single evaporation level
Enhanced cycles: • Double evaporation level • Pinch Point Smoother
• Double OFC
Optimized parameters
Evaporation temperature
Evaporation temperatures Evaporation temperatures + mass fractions Flash pressures
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Performance comparison: Tgeo 125-150°C
Comparison at constant minimum ∆Tpp : gross power
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Performance comparison: Tgeo 125-150°C
Comparison at constant minimum ∆Tpp : net power
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Performance comparison: Tgeo 125-150°C
Required “U·S”at constant minimum ∆Tpp
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Performance comparison:Tgeo=150°C
Comparison at constant “U·S” (i.e. at roughly the same heat exchanger cost)
Extended comparison, considering also propane and R134a supercritical cycles
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
The answer to the challenge: T-Q diagrams (U·S=const)
double flash
supercritical, propane
supercritical, R134a
two pressure level ORC
base, single pressure level ORC
single pinch point smoother
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Performance comparison:Tgeo=150°C
Comparison at constant “U·S” (i.e. at roughly the same heat exchanger cost)
Extended comparison, with supercritical cycles
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Performance comparison: Tgeo 125-150°C
Comparison at constant “U·S”
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Conclusions • The two pressure level emerges as the best solution, followed by the Pinch Point Smoother solution, giving a definite advantage compared to the base single pressure level ORC • The Pinch Point Smoother solution could be a feasible option for plant retrofit • Simple supercritical cycles have performance similar to the two pressure level cycle, though more demanding in terms of maximum cycle pressure • The double FLASH cycle solution seems not attractive, at least with HFC245fa in standard geothermal applications; it could however be attractive in special cases, e.g. with very high pressure geothermal fluids.
P. Bombarda
M. Gaia
C. Invernizzi
C. Pietra
Thank you for attention Paola Bombarda
[email protected] Gecos Group - http://www.gecos.polimi.it/
Politecnico di Milano